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I have been doing performance experiments with RFile. During the course of these experiments I noticed that RFile is not as fast at it should be in the case where index blocks are in cache and the RFile is not already open. The reason is that the RFile code copies and deserializes the index data even though its already in memory.
I made the following change to RFile in a branch.
- Avoid copy of index data when its in cache
- Deserialize offsets lazily (instead of upfront) during binary search
- Stopped calling lots of synchronized methods during deserialization of index info. The existing code use ByteArrayInputStream which results in lots of fine grained synchronization. Switching to an inputstream that offers the same functionality w/o sync showed a measurable performance difference.
These changes lead to performance in the following two situations :
- When an RFiles data is in cache, but its not open on the tserver.
- For RFiles with multilevel indexes with index data in cache. Currently an open RFile only keeps the root node in memory. Lower level index nodes are always read from the cache or DFS. The changes I made would always avoid the copy and deserialization of lower level index nodes when in cache.
I have seen significant performance improvements testing with the two cases above. My test are currently based on a new API I am creating for RFile, so I can not easily share them until I get that pushed.
For the case where a tserver has all files frequently in use already open and those files have a single level index, these changes should not make a significant performance difference.
These change should result in less memory use for opening the same rfile multiple times for different scans (when data is in cache). In this case all of the RFiles would share the same byte array holding the serialized index data.
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